Method and system for demand defrost control on reversible heat pumps

Abstract

A control processor controls an outdoor coil defrost cycle on a reversible heat pump by continuously monitoring the difference between the outdoor coil temperature and the outdoor air temperature. When specified preconditions are met and this difference exceeds a target value, the defrost cycle is initiated.

What is claimed is:  
     
       1. A method for controlling a coil defrosting cycle in a reversible heat pump system using a refrigerant cycle, comprising the steps of: 
       monitoring an outdoor coil temperature of an outdoor coil of said heat pump system;  
       monitoring an outdoor air temperature in a vicinity of said outdoor coil;  
       determining a maximum allowable difference between said outdoor air temperature and said outdoor coil temperature; and  
       defrosting said coil when a dynamically determined difference between said outdoor air temperature and said outdoor coil temperature exceeds said maximum allowable difference for a predetermined amount of time and predetermined conditions of said system are met.  
     
     
       2. A method according to claim  1 , where in said step of defrosting occurs if an accumulated compressor-on time exceeds a specified amount of time and said predetermined conditions of said system are met. 
     
     
       3. A method according to claim  1 , where in said step of determining said maximum allowable difference includes: 
       determining a minimum difference between said outdoor air temperature and said outdoor coil temperature;  
       adding a first constant to a first percentage of said outdoor air temperature and subtracting said minimum difference to obtain a first possible value;  
       adding a second constant to a second percentage of said outdoor air temperature and subtracting said minimum difference to obtain a second possible value; and  
       selecting a larger of said first and second possible values as said maximum allowable difference.  
     
     
       4. A method according to claim  3 , wherein said first constant is 4.0 degrees F., said first percentage is 60 percent, said second constant is 15 degrees, and said second percentage is 12.5 percent. 
     
     
       5. A method according to claim  3 , wherein said first constant is 7.0 degrees, said first percentage is 50 percent, said second constant is 15 degrees, and said second percentage is 12.5 percent. 
     
     
       6. A method according to claim  1 , further comprising the steps of: 
       monitoring said outdoor coil temperature; and  
       stopping said step of defrosting when said outdoor coil temperature exceeds a predetermined temperature or when a period of said defrosting exceeds a predetermined time.  
     
     
       7. A system for controlling a coil defrosting cycle in a reversible heat pump system using a refrigerant cycle, comprising: 
       means for monitoring an outdoor coil temperature of an outdoor coil of said heat pump system;  
       means for monitoring an outdoor air temperature in a vicinity of said outdoor coil;  
       means for determining a maximum allowable difference between said outdoor air temperature and said outdoor coil temperature; and  
       means for defrosting said coil when a dynamically determined difference between said outdoor air temperature and said outdoor coil temperature exceeds said maximum allowable difference for a predetermined amount of time and predetermined conditions of said system are met.  
     
     
       8. A system according to claim  7 , wherein said means for defrosting includes means for defrosting said coil if an accumulated compressor-on time exceeds a specified amount of time and said predetermined conditions of said system are met. 
     
     
       9. A system according to claim  7 , wherein said means for determining said maximum allowable difference includes: 
       means for determining a minimum difference between said outdoor air temperature and said outdoor coil temperature;  
       means for adding a first constant to a first percentage of said outdoor air temperature and subtracting said minimum difference to obtain a first possible value;  
       means for adding a second constant to a second percentage of said outdoor air temperature and subtracting said minimum difference to obtain a second possible value; and  
       means for selecting a larger of said first and second possible values as said maximum allowable difference.  
     
     
       10. A system according to claim  9 , wherein said first constant is 4.0 degrees F., said first percentage is 60 percent, said second constant is 15 degrees, and said second percentage is 12.5 percent. 
     
     
       11. A system according to claim  9 , wherein said first constant is 7.0 degrees, said first percentage is 50 percent, said second constant is 15 degrees, and said second percentage is 12.5 percent. 
     
     
       12. A system according to claim  7 , further comprising: 
       means for monitoring said outdoor coil temperature; and  
       means for stopping said step of defrosting when said outdoor coil temperature exceeds a predetermined temperature or when a period of said defrosting exceeds a predetermined time.